1. Field of the Invention
The present invention is generally directed to semiconductor lithography tools. In particular, the present invention is directed to chucks for precisely holding objects in position within lithography tools, while allowing for greater control system bandwidths.
2. Background
As semiconductor devices grow increasingly smaller, the demands on lithography tools increase. Specifically, chuck position tolerances decrease, which causes demands on the lithography tool positioning control systems to increase. For example, modern semiconductor geometries require chuck tracking and positioning to be accurate to 10 nanometers or better. In the past, chucks have been made from materials with relatively high thermal expansion, such as silicon carbide. These materials successfully met the less stringent requirements of their era without any thermal expansion compensation. Conventionally, however, their use is disfavored.
The state of the art is to manufacture the precision portions of lithographic stages, such as wafer and reticle chucks, from ultra low expansion materials. Conventionally, ultra low expansion materials are used in order to keep the thermal strain low. Low thermal strain is desirable for improving the positioning accuracy of the chuck, which holds a reticle or wafer during scanning operations. This is because uncompensated changes in stage dimensions caused by temperature variations increase positioning uncertainty of the object being scanned. Low thermal strain is also desirable because it reduces thermal distortion of reticles and wafers by constraining them with chucks that expand less than the objects being constrained.
Nonetheless, there are two major problems associated with the material properties of ultra low expansion materials conventionally used for chucks in precision lithographic stages. First, they have mediocre specific stiffness. In other words, they are not very stiff for their density. Eigenfrequencies, or resonant frequencies, of these conventional materials are proportional to the square root of their specific stiffness. The lowest chuck resonant frequency, also known as its fundamental frequency is a limiting factor in selecting the frequency (or bandwidth) of the control system for the lithography tool because a control frequency at or above the chuck's fundamental resonant frequency may cause the chuck to vibrate. This compromises the dynamic performance of critical stage components, and adversely affects overall scanning performance.
Second, ultra low expansion materials have a very low thermal conductivity. As a result, localized heating can occur as heat is not evenly spread through the chuck. Poor heat dissipation limits the amount of heat that can be applied by motors, actuators and the like. Poor heat dissipation also tends to reduce system performance, as heavy cooling and heat shielding components are needed in other moving portions of the stage to prevent heat from affecting the chuck.
It is thus clear that an ultra low expansion material with high specific stiffness and high thermal conductivity would be extremely desirable for making critical components of precision lithographic stages.